1 82 STUDIES FOR STUDENTS 



not open its mouth wide enough to bring the teeth into play. 

 It has been suggested that the teeth were used to aid it in 

 climbing trees. 



The existing genera of the cats were developed in the Plio- 

 cene and Pleistocene. 



Pinnipeda. — The water living carnivores seem to have sprung, 

 as has been suggested, from the Creodont Patriofelis. The 

 existing families can be traced back as far as the Pliocene but 

 beyond that there is nothing to connect them with the early 

 forms. Fragmentary skeletons of seals and walruses have been 

 found in the Pliocene of Europe and America. 



Insectivora. — The Insectivores are among the least changed 

 of all the Mammals from their prototypes of the earliest Ter- 

 tiar}^. The brain, carpus and dentition all present characters that 

 are found in the Creodonts. Many of the existing families are 

 found in the Eocene. 



Chiroptera. — The bats are known from deposits as early as 

 the Eocene in Europe and America, but nothing is known of 

 their ancestry; the fossil genera, including those extinct, are very 

 similar to the living forms. 



Rodentia. — The order is characterized by the absence of the 

 canine teeth, the development of two incisors in the upper and 

 the lower jaws as gnawing teeth which grow from persistent 

 pulps and the arrangement of the articular condyle so that the 

 lower jaw can slide backward and forward in the act of 

 grinding up the food. There are many primitive characters in 

 the group which is a remarkably persistent one, well-defined 

 rodents being known from the early Eocene. Three sub- 

 orders are known : the Tillodo?itia, Duplicidentata and the Sim- 

 plicidentata. 



Tillodontia. — These are forms that are known from the Eocene 

 deposits of Europe and America. In these the canines are still 

 preserved as rudiments, and there are sometimes more than the 

 single pair of incisors in the lower jaw. 



Esthyonx from the Wasatch and Bridger series of the United 

 States is the earliest form known. A fragmentary skull from 



